Extracting Power from Nuclear in Space

[wumpus]

28 minutes ago, shynung said:

If the reactor is powered down, it doesn't emit as much deadly radiation as it does under full power. If a NTR is powered down post-burn, it can be left still hot - the heat will radiate on its own, given enough time - without the crew needing to worry about deadly radiation, as long as the reactor components can withstand the temperature.

Should a heat engine (thermocouple) be plugged in at this state, the reactor would get cold fast, because while heat flows through the heat engine, the reactor isn't producing additional heat to compensate. Then, once the reactor temperature is the same as the radiator's, the heat engine stops generating power, and the reactor stops cooling.

Also, when you wrote 'fuel', you might meant 'propellant' - that's the stuff spewing out of the rocket nozzle. 'Fuel' in the context of nuclear thermal rocket would be the slug of U235 in the reactor itself.

A closed cycle reactor will have the same cooling system it used when operating after it is turned off, so that shouldn't be an issue.  When a NTR (or any nuclear reactor) is "turned off' the nuclear fuel will still be reacting as some of the fuel will be turned into isotopes that are still emitting neutrons and powering the reaction.  Simply "leaving it hot" is likely to cause a full meltdown*.  Also I wonder at the idea of powering down a reactor to limit radiation.  Normally I'd assume that lowering the output such the the reactor is "always on" would reduce the danger from radiation (assuming equal shielding), but in practice I'm less sure (ignoring strategies involving sequestering crew in highly shielded  areas during a burn).  I still think that cooling issues will drive a "always on" solution, cooling a reactor in space is non-trivial and the danger of coolant failure (abandon ship**) outweighs the danger of radiation.

Propellant is a good word.  I kept thinking "reaction mass" which also doesn't work in the context of a nuclear reactor.

* KSP's "source of all information" describes this in a recent series: https://www.youtube.com/watch?v=pWWjbnAVFKA It might be the next video, but it does include how the "secondary reactions" work and why the reaction continues after the control rods are full down.

** more likely eject the reactor.  This would leave you "dead in space", but at least have as much life support as possible and presumably a more comfortable life raft.

 

[shynung]

21 minutes ago, wumpus said:

A closed cycle reactor will have the same cooling system it used when operating after it is turned off, so that shouldn't be an issue.  When a NTR (or any nuclear reactor) is "turned off' the nuclear fuel will still be reacting as some of the fuel will be turned into isotopes that are still emitting neutrons and powering the reaction.  Simply "leaving it hot" is likely to cause a full meltdown*.  Also I wonder at the idea of powering down a reactor to limit radiation.  Normally I'd assume that lowering the output such the the reactor is "always on" would reduce the danger from radiation (assuming equal shielding), but in practice I'm less sure (ignoring strategies involving sequestering crew in highly shielded  areas during a burn).  I still think that cooling issues will drive a "always on" solution, cooling a reactor in space is non-trivial and the danger of coolant failure (abandon ship**) outweighs the danger of radiation

For a start, there are things such as bimodal nuclear reactors. This is the type of reactor that both propels a ship  by being a NTR, while also providing power through a secondary coolant loop. The reactor isn't completely turned off, merely kept warm, to provide power to the ship.

[PB666]

What I find amazing here is that we are repeatedly talking about Inefficient power systems working at a scale in space that no space agency has even come close to attempting. More amazing is that we hand wave the heat problem away like heat is easy enough to toss off the ship as fairy dust.

Lets have some equations here, you have 66MW of heat generation. The power drop of 8'K off the first heat exchanger.  . . . . . . . .make these things credible show the math. What is the size of the heat exchanger, how is the 13.5 MW of power generated (no steam obviously) . . . . . . Where are the equations for determining the size of the heat panels????

The claim in the OP is that this device can generate power from ambient heat in (whatever), this means the temperature drops and it sends current. (Sounds awfully like perpetual motion because it can establish a temperature gradient where none exists). But lets say it can do this with a very tiny gradient.

[magnemoe]

26 minutes ago, wumpus said:

A closed cycle reactor will have the same cooling system it used when operating after it is turned off, so that shouldn't be an issue.  When a NTR (or any nuclear reactor) is "turned off' the nuclear fuel will still be reacting as some of the fuel will be turned into isotopes that are still emitting neutrons and powering the reaction.  Simply "leaving it hot" is likely to cause a full meltdown*.  Also I wonder at the idea of powering down a reactor to limit radiation.  Normally I'd assume that lowering the output such the the reactor is "always on" would reduce the danger from radiation (assuming equal shielding), but in practice I'm less sure (ignoring strategies involving sequestering crew in highly shielded  areas during a burn).  I still think that cooling issues will drive a "always on" solution, cooling a reactor in space is non-trivial and the danger of coolant failure (abandon ship**) outweighs the danger of radiation.

Propellant is a good word.  I kept thinking "reaction mass" which also doesn't work in the context of a nuclear reactor.

* KSP's "source of all information" describes this in a recent series: https://www.youtube.com/watch?v=pWWjbnAVFKA It might be the next video, but it does include how the "secondary reactions" work and why the reaction continues after the control rods are full down.

** more likely eject the reactor.  This would leave you "dead in space", but at least have as much life support as possible and presumably a more comfortable life raft.

An reactor generate all the power because of the chain reaction who generate a lot of radiation, if you pull out the moderators you don't have an chain reaction just normal radioactivity. yes this will still heat the reactor, but hardly enough to boil water. Now one issue with an large power reactor is the size of the core, the core is huge and its hot so just shutting it down don't work on most designs, you have to keep cooling it. 
This happened at Fukushima, as they did not get more cooling water they had to vent some of it, they also got an hydrogen explosion because of overheated steam. 
An small nuclear reactor like the one in an submarine don't have this issue because of far lower thermal mass, you can shut it down and the structure will cool it enough.
However you would want to run an NTR as hot as possible so you would need to shut it off an keep cooling it for some time, this can be done with fuel or an extra cooling loop who can also run ship systems. 
Now if you run an ion engine you would probably run on pretty high power mostly limited by radiators, I like the idea, you get an pretty high trust with 800 s then you continue with say 8000 s later. Just for running ship systems you would not need much power. 

[PB666]

12 minutes ago, shynung said:

For a start, there are things such as bimodal nuclear reactors. This is the type of reactor that both propels a ship  by being a NTR, while also providing power through a secondary coolant loop. The reactor isn't completely turned off, merely kept warm, to provide power to the ship.

How is the waste gas accelerated. And my presumption with Space nuclear is that you start with alot of hydrogen and a little Deuterium. As the fusion reactor goes over time it generates deuterium from hydrogen which is then used to fuel the reactor. If you expel the reactants into space prior to purification you waste the neutrons.

Anyway how is the waste gas accelerated? rf? Lasers? Magnetism and Charge variance? As stated previously every metal has its breaking limit temperature, that temperature determines the ability to accelerate. To get higher than that you need to use EM or its surrogate charge propulsion and magnetism. In fusion you have to create a plasma and pressurize it, this can be used to expel the gas. THE BIG PROBLEM IS: you have to bring a power supply or battery along that can get this process going _and then_ after its completed you have to recharge. Are you using solar cells (for example while off-cycle kick burning out of orbit around a planet).  Constituitive space craft operations are nothing compared to the cost of sustaining and using MW of power in generation.

[magnemoe]

8 minutes ago, PB666 said:

What I find amazing here is that we are repeatedly talking about Inefficient power systems working at a scale in space that no space agency has even come close to attempting. More amazing is that we hand wave the heat problem away like heat is easy enough to toss off the ship as fairy dust.

Lets have some equations here, you have 66MW of heat generation. The power drop of 8'K off the first heat exchanger.  . . . . . . . .make these things credible show the math. What is the size of the heat exchanger, how is the 13.5 MW of power generated (no steam obviously) . . . . . . Where are the equations for determining the size of the heat panels????

The claim in the OP is that this device can generate power from ambient heat in (whatever), this means the temperature drops and it sends current. (Sounds awfully like perpetual motion because it can establish a temperature gradient where none exists). But lets say it can do this with a very tiny gradient.

My reaction too, you can not generate power from heat without an cold and an hot side who is connected somehow. 
And yes this is an issue, no problem running an clock of the heat difference between front and back during the winter, however clock will stop working in the tropic. 
it will not work in an pacemaker as its no temprature gradient inside your body

[PB666]

7 hours ago, magnemoe said:

An reactor generate all the power because of the chain reaction who generate a lot of radiation, if you pull out the moderators you don't have an chain reaction just normal radioactivity. yes this will still heat the reactor, but hardly enough to boil water. Now one issue with an large power reactor is the size of the core, the core is huge and its hot so just shutting it down don't work on most designs, you have to keep cooling it. 
This happened at Fukushima, as they did not get more cooling water they had to vent some of it, they also got an hydrogen explosion because of overheated steam. 
An small nuclear reactor like the one in an submarine don't have this issue because of far lower thermal mass, you can shut it down and the structure will cool it enough.
However you would want to run an NTR as hot as possible so you would need to shut it off an keep cooling it for some time, this can be done with fuel or an extra cooling loop who can also run ship systems. 
Now if you run an ion engine you would probably run on pretty high power mostly limited by radiators, I like the idea, you get an pretty high trust with 800 s then you continue with say 8000 s later. Just for running ship systems you would not need much power. 

Right, fission just doesn't shut down! I was going to mention this. In a fission reactor a high percentage of the fuel is undergoing a low rate of decay, water and graphite are moderators, but even as one removes the rods or raises the graphite, the downstream short-lives isotopes are still decaying for days after.

Fusion is different, in a fusion reactor reactants are pulsed into the reactor, undergo fusion and then expelled. In this process they lose heat. There are three primary products, helium, dueterium, and tritium (half-life is 12 years),

7 hours ago, magnemoe said:

My reaction too, you can not generate power from heat without an cold and an hot side who is connected somehow. 
And yes this is an issue, no problem running an clock of the heat difference between front and back during the winter, however clock will stop working in the tropic. 
it will not work in an pacemaker as its no temperature gradient inside your body

On a quantum scale this can happen, what would happen on graphene is that it would cool and the ripples would decrease intensity and stop, therefore there would need to be a heat source to restore the rippling.

Edited November 26, 2017 by PB666

[shynung]

17 minutes ago, PB666 said:

How is the waste gas accelerated. And my presumption with Space nuclear is that you start with alot of hydrogen and a little Deuterium. As the fusion reactor goes over time it generates deuterium from hydrogen which is then used to fuel the reactor. If you expel the reactants into space prior to purification you waste the neutrons.

This is a fission-based system. I can't find a detailed fusion power reactor design anywhere - the ones I do find are propulsion systems, not power generation systems.

27 minutes ago, PB666 said:

More amazing is that we hand wave the heat problem away like heat is easy enough to toss off the ship as fairy dust.

To be fair, it's not that difficult. Get the radiators hot enough, and they'll spew the heat away. The problem is engineering the radiator.

Edited November 26, 2017 by shynung

[PB666]

Fission based expulsion systems typically have low ISP. 1st the Power/mass is low and second the end products are heavy and do not gain much speed.

[shynung]

1 minute ago, PB666 said:

Fission based expulsion systems typically have low ISP. 1st the Power/mass is low and second the end products are heavy and do not gain much speed.

IDK, 800-900 seconds of ISP on LH2 typically claimed on NTR performance is pretty good for me. Especially since chemical systems struggle to get over 400 seconds without using 'exotic' propellants (i.e. ones that do horrendous things to humans - fluorine and their ilk). Even when using water as propellant, 400 seconds is still possible.

[wumpus]

6 minutes ago, shynung said:

IDK, 800-900 seconds of ISP on LH2 typically claimed on NTR performance is pretty good for me. Especially since chemical systems struggle to get over 400 seconds without using 'exotic' propellants (i.e. ones that do horrendous things to humans - fluorine and their ilk). Even when using water as propellant, 400 seconds is still possible.

This is certainly reasonably high for any travel inside the solar system, although you probably won't try any "fast Mars" trips (faster than Hohmann).  Interstellar isn't going to happen with 3 (or even 4) digit Isp.  I was told that the 800 Isp number was from the 1960s, and that materials science could provide better now (although I suspect that means either ejecting spent fuel or paying in cooling/dry mass so much to erase the Isp gains).

55 minutes ago, shynung said:

For a start, there are things such as bimodal nuclear reactors. This is the type of reactor that both propels a ship  by being a NTR, while also providing power through a secondary coolant loop. The reactor isn't completely turned off, merely kept warm, to provide power to the ship. [image deleted]

Obviously you would want to turn waste heat into power, if only to avoid having to radiate it.

Have you ever considered just how you would balance an ion system with a NTR?  The concept seems completely insane (on the other hand, if you where using VASIMR you could at least scale up your power and use the same propellant) as the thrust of the ion would be completely in the noise of the NTR, while the Isp of the NTR would be such that if the ion was worth using, you would never use the NTR.  Ions would only make sense if they were the most efficient means of transmitting energy (and thus heat) into the cosmos (I'd try LEDs first, or perhaps microwaves).

[shynung]

1 minute ago, wumpus said:

Have you ever considered just how you would balance an ion system with a NTR?  The concept seems completely insane (on the other hand, if you where using VASIMR you could at least scale up your power and use the same propellant) as the thrust of the ion would be completely in the noise of the NTR, while the Isp of the NTR would be such that if the ion was worth using, you would never use the NTR.  Ions would only make sense if they were the most efficient means of transmitting energy (and thus heat) into the cosmos (I'd try LEDs first, or perhaps microwaves).

The ion propulsion system on the diagram was there to provide ullage, trajectory corrections maneuver, and other low-load situations. NTR is used to cross the Van Allen belt quickly from LEO, and for Mars capture burn. This is done to reduce mass needing to be launched to LEO.

More on the technical paper here.

[wumpus]

24 minutes ago, shynung said:

The ion propulsion system on the diagram was there to provide ullage, trajectory corrections maneuver, and other low-load situations. NTR is used to cross the Van Allen belt quickly from LEO, and for Mars capture burn. This is done to reduce mass needing to be launched to LEO.

More on the technical paper here.

I don't think I've seen a paper with as much handwaving before.

Presumably the "scalable Brayton units" could be powered by the NTR in some sort of closed figuration (presumably the radiation from the fuel), but this isn't covered at all, neither the amount of power needed by the ion systems for such things nor the power generated by the NTR (either firing propellant or not.  Presumably they were mostly expected to work when not firing propellent and this isn't covered at all).  Especially missing was any idea of the size needed for the "brayton cooling" bit: the ISS uses radiators roughly 1/10th the mass of the solar panels, but here (which presumably uses vastly more power than the ISS) such cooling is an afterthought.

I still need to work out how many Pe kicks you can do between crossing the Van Allen belt and the burn beyond escape (presumably to Mars or the asteroid belt), but I suspect it is either one (a few km/s or so) or zero (it still does wonders in cutting down the thrust needed).

[DDE]

4 hours ago, shynung said:

Also, when you wrote 'fuel', you might meant 'propellant' - that's the stuff spewing out of the rocket nozzle. 'Fuel' in the context of nuclear thermal rocket would be the slug of U235 in the reactor itself.

No, I suspect they actually meant dumping the reactor, or a major chunk thereof. Early NERVAs were rated for one burn anyway, hence multiple NERVA stages in some designs.

2 hours ago, magnemoe said:

An reactor generate all the power because of the chain reaction who generate a lot of radiation, if you pull out the moderators you don't have an chain reaction just normal radioactivity.

In hours to days. These things take quite a while to throttle; that's why its likely any atomic rocket will have a Chief Engineer between the pilot and the actual reactor controls.

[wumpus]

23 hours ago, DDE said:

No, I suspect they actually meant dumping the reactor, or a major chunk thereof. Early NERVAs were rated for one burn anyway, hence multiple NERVA stages in some designs.

In hours to days. These things take quite a while to throttle; that's why its likely any atomic rocket will have a Chief Engineer between the pilot and the actual reactor controls.

Er yes.  Dumping the fuel (rods/pebbles/whatever) after each burn removes a massive cooling issue.  Not only will the unit remain "hot" for "hours to days", but the reaction will be going close to full speed for several minutes.  This becomes a huge problem as efficient burns should be done in minutes at a time (for Oberth.  You also need to get out of the Van Allan belts quickly, but I suspect that can work with the isotope-fed overtime), so using propellant to cool the fuel becomes wildly inefficient.

Unfortunately this leaves you with a radioactive mass of molten metal (regardless of the container you ejected).  You probably want to send a "fuel wrangler" probe to follow the molten radioactive melal and guide it through a slingshot maneuver into a "graveyard [solar] orbit".  This would obviously be trickiest for capture burns (presumably the burn wouldn't fully capture and you would still need aerocapture for the spacecraft and a quick maneuver with the fuel pad elsewhere).

[DDE]

8 minutes ago, wumpus said:

Unfortunately this leaves you with a radioactive mass of molten metal (regardless of the container you ejected).  You probably want to send a "fuel wrangler" probe to follow the molten radioactive melal and guide it through a slingshot maneuver into a "graveyard [solar] orbit".  This would obviously be trickiest for capture burns (presumably the burn wouldn't fully capture and you would still need aerocapture for the spacecraft and a quick maneuver with the fuel pad elsewhere).

Shouldn’t a simple rocket-assisted retrograde core dump early in flight already send it way off course?

Edited November 27, 2017 by DDE

[wumpus]

Just now, DDE said:

Shouldn’t a simple retrograde core dump early in flight already send it way off course?

It should, but remember the orbit still has to intersect where it was originally dumped.  In both escape and capture burns, that will put it in an orbit that intersects with a planetary orbit.  Eventually it will come down.  I really can't tell if that puts the fuel in "depleted uranium levels" or "turned to lead levels" before it is finally captured by the planet, but it is still an embarrassing place to leave a slug of melting radioactive metal.

[PB666]

Dump it after apogee surpasses 1.5 million miles it will wander around the solar system a bit. We can have a media storm about when it will come crashing back to earth, you could also dump it on a course that causes it to collide with the moon.

800-900 ISP is kind of poor, NTRs have very heavy engines and they run on hydrogen.

[DDE]

1 hour ago, PB666 said:

Dump it after apogee surpasses 1.5 million miles it will wander around the solar system a bit. We can have a media storm about when it will come crashing back to earth, you could also dump it on a course that causes it to collide with the moon.

800-900 ISP is kind of poor, NTRs have very heavy engines and they run on hydrogen.

Or you can have triple the ISP and cause the spent fuel to reach solar escape velocity, or, the opposite, drop into the sun.

Just use an open-cycle gas-core.

Edited November 27, 2017 by DDE

[shynung]

4 hours ago, DDE said:

Just use an open-cycle gas-core.

Was about to point this out. Yes, gas-core rockets can reach 4 digit ISP, and they also eject used nuclear fuel along with propellant. Of course, that was spent nuclear fuel that could have been reprocessed to recover the unfissioned U235/freshly-made Pu239. It's possible to use a closed-cycle gas-core rocket, but that means taking a hit on ISP, and needing to build a more complicated rocket.

[DerekL1963]

On 11/26/2017 at 11:22 AM, DDE said:

In hours to days. These things take quite a while to throttle; that's why its likely any atomic rocket will have a Chief Engineer between the pilot and the actual reactor controls.

o.0  No, try more like "seconds to tens of seconds (at worst)" - changes in moderation take effect fast (hence Chernobyl), and in reactors are really only limited by thermal inertia.

[DDE]

1 hour ago, DerekL1963 said:

o.0  No, try more like "seconds to tens of seconds (at worst)" - changes in moderation take effect fast (hence Chernobyl), and in reactors are really only limited by thermal inertia.

A quick glance through the apex of reliability, Wikipedia, suggests the events too place over the timeframe of tens of minutes. From what I heard full nuclear plant shutdown/start-up requires up to a day.

[shynung]

3 hours ago, DDE said:

From what I heard full nuclear plant shutdown/start-up requires up to a day.

I'm guessing this might be a terrestrial nuclear power plant. That long start-up/shutdown time might be to thermal inertia to the system as a whole - reactor, coolant, the works. AFAIK if enough neutrons are absorbed to stop further chain reaction, the remaining fuel goes to natural decay not too long afterwards.

[PB666]

7 hours ago, DDE said:

A quick glance through the apex of reliability, Wikipedia, suggests the events too place over the timeframe of tens of minutes. From what I heard full nuclear plant shutdown/start-up requires up to a day.

Greater than 6% of the power of a nuclear power plant comes from fission product isotopic decay.

Reading the posts . . . there seems to be some confusion between isotopic decay and neutron initiated fission.

To understand how a fission reactor works take a long thin yard(meter) stick and try to balance on the blade of a very sharp knife. Any movement to one side or the other and the yard stick falls off. Fission of unpurified uranium is not spontaneous, yes it occurs but only slowly. To increase the rate of reaction fast neutrons need to be slowed by water or graphite. However once you get the reaction going you want to moderate, oddly this is also done by water (circulation in the reactor) and by control rods that contain neutron absorbents. The control features of a reactor broaden  "the edge". If these features are removed the knife blade gets very thin and one can easily step into a prompt critical situation. If one moves to many control features into a reactor you can go the other way due to ¹³⁶Xe poisoning.

Here is a clear prompt critical situation. https://en.wikipedia.org/wiki/SL-1

Chernobyl was a mixed situation, but the 33+ GW explosion that took place was the consequence of poor fission modulation of a subset of pellets within the reactor, part of this was do to poor reactor design (this reactor design allow the build up of nucleated steam pockets) that was exacerbated by poor power to the cooling pumps that were required during reactor shut-down (as this was a shut down test and not a full shut down the exact qualification is ambiguous). The steam that built up around pellets that were not modulated by control rods allowed the pellets to go prompt critical, even though this did not include the entire reactor, it included enough of the reactor to create a catastrophic, epic, steam explosion. The heat of the  <190 MW contributed by isotopic decay helped in that the improper cooling that lead to the disaster the isotopic decay was contributing the steam-pocket generation potential. But the bad in all of this was there were some pellets that were too far from the control rods at the time and although they were a small subset of all the uranium, there was sufficient number to create a local prompt critical, of the 33+ GW of power generated less than 1% was from decay heat. THis is to say that Chernobyl could have happened in an ever so slightly different set of circumstances without decay heat. The difference between SL-1 was during its prompt critical the proportion was so great that all the water immediately boiled and threw the uranium over a wide area completely stopping all reaction. At Chernobyl the fraction was small enough to preserve the Uranium which then melted basically to the core and created the situation we see today. The people who died at chernobyl primarily died from the secondary effects of a reactor breach (one man died from being scalded by reactor water), at SL1 the people died from the primary effects of being at point blank range from a prompt-critical explosion, (i.e being impaled by a control rod).

Fukushima was almost entirely caused by the decay heat. In this is to say if your reactor has a 6% off power rating and your reactor can ambiently remove 1% of that power, that over time (say 24 hours) the heat of the decay will cause the reactors to blow off steam and hydrogen and eventually melt down.

Why this has to do with space. Steam generation is the most efficient way of capturing the power generated by heat in the reactor. In general the water traps the heat, and the steam takes the power from that water and delivers it to one or two generators in a series and finally dumps the remnant to a cooling system before it is pumped back to the reactor. The steam/water separator depends on inertia and gravity. In addition nucleated steam on the pellets can become stable without gravity to remove the steam. For this reason steam reactors in space are highly limited in size and output.

 

[wumpus]

So it looks like the choices for dealing with post-burn heat are:

* Open-cycle cooling: send propellent through the system, possibly into a smaller bell to act efficiently at 6-10% propellant mass, possibly switching to a non-propellant with either higher thermal mass/mass ratio or would suck heat better during a phase change (or two if using a solid).  Ideally this can be ISRU ice, but still it involves mass.
* Closed cycle cooling: This is highly complex and has most of the issues PB666 just described.  Expect *lots* of dry mass needed to do this.
* Dumping the fuel: obviously you need all the fuel mass each burn, but expect it not to be much more massive than the cooling mass needed in open-cycle designs.  Just don't count on any ISRU uranium (unlike the ice example).

The major advantage here for dumping fuel is simplicity (it probably requires less total mass than closed cycle cooling as well).  If you have a meltdown (and I'd expect it to happen when leaving Earth or possibly leaving for Earth with failing ISRU material) you aren't going  to be doing a capture burn and can expect to be lost in space in some sort of transfer orbit oscillating between Earth orbit and some other orbit.  Trying to move the fuel into a graveyard orbit might be avoided simply to avoid failure: for some reason people will object if you try to avoid a .001% chance of Earth being hit by radioactive slag and fail vs. simply assume that "it is all right" and ignore it.